Directional couplers are widely used in front end module (FEM) products, such as radio transceivers, wireless handsets, and the like. For example, a directional coupler can be used to detect and monitor radio-frequency (RF) output power. When an RF signal generated by an RF source is provided to a load, such as to an antenna, a portion of the RF signal can be reflected from the load back toward the RF source. An RF coupler can be included in a signal path between the RF source and the load to provide an indication of forward RF power of the RF signal traveling from the RF source to the load and/or an indication of reverse RF power reflected from the load. RF couplers include, for example, directional couplers, bi-directional couplers, multi-band couplers (e.g., dual band couplers), and the like.
Referring to FIG. 1, an RF coupler 100 typically has a power input port 102, a power output port 104, a coupled port 106, and an isolation port 108. The electromagnetic coupling mechanism, which can include inductive or capacitive coupling, is typically provided by two parallel or overlapped transmission lines, such as microstrips, strip lines, coplanar lines, and the like. The transmission line 110 extending between the power input port 102 and the power output port 104 is termed the main line and can provide the majority of the signal from the power input port 102 to the power output port 104. The transmission line 112 extending between the coupled port 106 and the isolation port 108 is termed the coupled line and can be used to extract a portion of the power traveling between the power input port 102 and the power output port 104 for measurement. When a termination impedance 114 is presented to the isolation port 108 (as shown in FIG. 1), an indication of forward RF power traveling from the power input port 102 to the power output port 104 is provided at the coupled port 106. Similarly, when a termination impedance is presented to the coupled port 106, an indication of reverse RF power traveling from the power output port 104 to the power input port 102 is provided at the isolation port 108. The termination impedance 114 is typically implemented by a 50 Ohm shunt resistor in a variety of conventional RF couplers.